This invention relates to systems for inspecting and characterizing Micro-Electro-Mechanical Systems (MEMS), especially to systems for inspecting, testing and characterizing MEMS that contain elements that move during operation.
MEMS have been in widespread use for many applications, such as accelerometers, pressure sensors and ink jet printer heads. In optical communication, MEMS are finding applications in attenuators, tunable lasers, tunable filters and optical switches. All of these applications employ members that move during operation. For MEMS used in fiberoptic communication which employs members that move during operation, it is important to be able to measure characteristics of the movable members in MEMS such as friction, sticktion, stiffness, hysteresis, operating range, frequency response, deflection angle and reflectivity at wavelengths used for fiberoptic communications, such as at 1.31 and 1.55 microns. Thus, in optical switching applications, for example, while the mechanical properties of the movable members in MEMS define parameters such as speed of switching and crosstalk, the optical properties of these members such as reflectivity also affect performance of the switches since the optical properties are directly related to factors such as insertion loss.
Conventional methods for characterizing MEMS employ either vision system or interferometers. Vision systems capture the movement of MEMS through a sequence of frames and extract motional parameters from image processing algorithms applied to the captured image frames. Interferometers detect motion from optical path changes.
Vision systems are generally less sensitive and can only operate at lower bandwidth due to the limited frame rate of image sensors. Algorithms used in vision systems tend to be complicated and expensive, and the vision data obtained carries minimal useful information. To attain useful information, large amounts of data must be obtained in vision systems and processed. Interferometers, on the other hand, are most sensitive to motion. However, interferometers are generally also sensitive to environmental disturbance such as vibration and temperature variations. The tilt of optical components in MEMS during operation can be as large as 20 degrees. Therefore, the sensitivity offered by interferometers and the cost thereof may not be justified for MEMS applications. Furthermore, to this date there is no satisfactory solution for simultaneously measuring both the mechanical and optical properties of MEMS.
It is therefore desirable to provide a system for measuring MEMS where the above-described difficulties are avoided or alleviated.
This invention is based on the observation that the above objectives can be achieved by detecting the reflection of an illumination beam from a movable component in MEMS by means of a detector when the component is at different positions as a result of its movement or tilt in order to determine the characteristics of the MEMS, such as mechanical and/or optical characteristics.
When the invention is implemented in a stand-alone apparatus, the illumination beam is provided by a source directed towards the surface of a movable MEMS system component. Preferably, an external stimulus causes relative tilting between the beam and the surface about a pivot, or about one or more axes. The reflection of the illumination beam from the surface at different positions is collected by an optical collector and then detected by the detector.
Preferably, the stimulus causes the surface of the MEMS system component to oscillate at at least one frequency in order to measure the frequency response of the MEMS system component.
Preferably, the detector is a two-dimensional detector such as a position sensitive detector or a one- or two-dimensional photodetector array that measures the position of the reflected beam along one axis or two axes that are transverse to each other. Also preferably, the system can simultaneously measure the reflectivity of the MEMS system component surface and any defects thereon.